As of recent, various types of variable-damping-force damper used in automotive suspensions, capable of variable control of damping force in stepwise or non-stepwise, have been developed. Known mechanisms to change damping force include a mechanical type where the area of an orifice provided to a piston is changed by a rotary valve, and a magneto-rheological fluid (hereinafter, “MRF”) type which uses MRF as an operating oil, in which the viscosity of the MRF is controlled by a magnetic fluid valve provided to the piston. Enabling control to change the damping force of the damper in accordance with the state in which the vehicle is being driven can improve operation safety and comfort of the ride.
One known technique to improve comfort of the ride is skyhook control, based on the skyhook theory. Skyhook control performs ride quality control (sprung damping control) requires detection of sprung speed, in order to set a target damping force to suppress vertical direction movement of the sprung mass. Even if the area of the orifice and the viscosity of the MRF is constant, damper properties exhibit change in damping force according to the stroke speed. Accordingly, the stroke speed, which is the relative displacement speed between the sprung mass and unsprung mass, needs to be detected in order to perform skyhook control.
Suspension control devices performing skyhook control according to the related art required vertical G sensors or stroke sensors to be mounted for each wheel, to detect the vertical speed of the sprung mass and the stroke speed. However, stroke sensors are attached in or nearby the wheel wells, thus securing installation space is difficult. In order to deal with this problem, there has been proposed a suspension control device which controls the damping force of the dampers without installing stroke sensors. This proposal describes calculating the relative displacement speed of sprung and unsprung masses, from the wheel speed fluctuation amount, and using the calculated relative displacement speed and so forth to control the damping force (see Japanese Unexamined Patent Application Publication No. 6-48139).
However, the suspension device according to Japanese Unexamined Patent Application Publication No. 6-48139 calculates the relative displacement speed between the sprung mass and unsprung mass. This is done taking advantage of the fact that when the wheels move vertically relative to the body in accordance with the geometry of the suspension, the wheel speed fluctuates due to the wheels moving relative in the longitudinal direction of the vehicle in accordance with the caster angle. This means that when the caster angle set to the suspension is small or zero, the relative displacement speed cannot be calculated, or the calculation precision deteriorates. Also, sprung speed is detected using vertical G sensors additionally installed, and this has been one factor in the high cost of suspension control devices which perform skyhook control.
Moreover, the relative displacement speed between the wheels and body is calculated with the suspension device in Japanese Unexamined Patent Application Publication No. 6-48139 based on wheel speed, meaning that if the wheels slip, control based on the relative displacement speed becomes inaccurate, thus there is the concern that vehicular behavior may become unstable. An arrangement may be conceived that the control current of the dampers is fixed when a vehicle behavior control device configured to stabilize vehicular behavior in cases of the wheels slipping or the like. However, this arrangement has problems in that fixing the control current when the damping force of the dampers is small, insufficient damping force may not be able to maintain the vehicle in an appropriate posture, and the unsprung components may thrash.